Piezoelectric measuring elements are used in a plurality of applications, for example in pressure, force, elongation or in acceleration sensors. During a measurement a body made of piezoelectric material is subjected to a mechanical strain whereby electrical charge is generated at particular surfaces of the body. This charge is collected and led to a cable connection by means of electrodes. Eventually, the cable transmits the charge to an amplifier which may also be a preamplifier.
Basically, there are two different possibilities for the transmission of the charge to the amplifier, namely using a coaxial cable or using a two conductor cable. If a coaxial cable is used the measurement data are transmitted from the first electrode to the inner conductor while the other electrode can lie on mass potential and can be connected to the shielding which protects the measurement line from external interfering fields. In addition, a second shielding can be provided by means of a triaxial cable. In contrast, if a two conductor cable is used the two electrodes are connected with two leads of a cable which itself can be shielded as a whole in a single or dual manner. At the amplifier, the charge difference between the two leads is evaluated. Since the leads are exposed to the same interfering fields no alteration in the charge difference results from an interfering field.
Applications using the two conductor technology are for example measuring elements having one or more piezoelectric plates with longitudinal effect. Longitudinal effect means that the charge is generated at those surfaces at which the force is introduced. These surfaces are most often metallised to collect the charge. Metallisations with the same polarization are each connected to one electrode and the electrode is provided with one of the two conductors. Since the metallised surfaces are under load a good contact of adjacent surfaces is always ensured.
Applications using the coaxial technology also comprise measuring elements having one or more piezoelectric plates with longitudinal effect the electrodes of which are connected to the inner conductor and the shielding, respectively. On the other hand, also the use of bars with transverse effect made of piezoelectric material with the coaxial technology is known. Transverse effect means that the surfaces at which charge is generated under load are positioned perpendicularly to the surfaces at which the load is introduced. EP 1 283 552 describes an application of this type comprising one, WO 2005/026678 an application comprising several bar crystals with transverse effect. In these cases, the electrodes which collect the charges are electrically connected to metallisations on the force-introducing surfaces ensuring a good contact for collecting the charge.
The advantage of piezoelectric bars with transverse effect over such plates with longitudinal effect lies in their sensitivity and in the cost-performance ratio. To obtain the same sensitivity which can be achieved with a thin bar with plates a plate stack of many plates must be used which is much more expensive than the bar and in addition also considerably high. To further increase the stability and the sensitivity of measuring elements containing bars also several or thinner bars can be used as mentioned in WO 2005/026678. In this application the electrodes are also electrically connected to metallisations on the two force-introducing surfaces of the piezoelectric bars whereby a good contact is obtained.
Another application using several bars is also known from FIG. 2 of EP 1 283 552. There, three bars are arranged in a triangle wherein in the interior of the triangle a spring receives the charge as the first electrode while the spring at the same time presses the external walls of the crystals to a metallic wall which is the second electrode.
The advantage of the two conductor technology compared to the coaxial technology is the ground insulation. Every measuring element with two conductor technology is inevitably ground insulated while a measuring element using coaxial technology often has an electrode on the mass potential. Only by an elaborate assembly using insulation rings and shields of the measuring element is a ground insulation obtained. This is for example requisite in the case of combustion engines due to the long signal leads to prevent interference by compensating voltages in ground connections, so called ground loops. It must be noted that the use of a sensor in a combustion engine requires a temperature stability of at least 300° C. This necessitates particular elements withstanding these temperatures.
It is not possible to use the known bars with transverse effect in two conductor technology since the individual electrodes due to the construction are subject to different interfering fields thus biasing the measurement signal.